In a numerically controlled machine tool (hereinafter called "NC machine tool"), a displacement error is produced from time to time between the spindle and a workpiece according to the running conditions, thereby causing a machining accuracy to be changed. A drive section is one of the factors that may cause such dynamic errors. The drive section is roughly divided into the spindle head and feed drive section. Thermal displacement due to heat generation at the bearings, gear drive system, etc. In line with the spindle rotation in the spindle head and lost motions in reversing of the feed direction in addition to thermal displacement due to heat generation of ball screws in the feed drive section adversely influence the dynamic errors to a large extent.
As regards the accuracy errors due to thermal displacement, such a system is currently employed, in which the displacement quantity of the spindle head and ball screws is measured, stopping the spindle rotation and feed drive while the machine is running or during machining, a correcting quantity of thermal displacement is inputted in numerical control equipment (hereinafter called "NC equipment") on the basis of the result of measurement, and a reference zero point correction is performed from time to time. However, not only is the error correcting through measurement not efficient but also the measurement accuracy by devices, measurement errors, etc. will be reflected in the correcting quantity as they are. Therefore, there are many cases where it is difficult to secure the reliability.
The feed drive in an NC machine tool is such that generally rotation of servo motors of each feed drive axis is converted to linear movements of table, etc., via a feed drive system such as ball screws. However, there are cases where an error is generated between the stop position by positioning in the positive orientation to a specified position and the stop position by positioning in the negative orientation. This error is generally called "lost motion". This lost motion is produced by complicated influences due to backlashes, elastic deformation of the feed drive system, friction of guideways by which a table, etc. is guided, etc. When, in a positioning movement of the feed drive axis, reversing the orientation of feed after the table, etc. is moved in the positive or negative orientation, a time lag is produced by this lost motion between an instruction to the servo motor from the NC equipment and the actual movement of the table, etc. When carrying out an interpolation by simultaneous plural axes, for example, changing the quadrant in a circular interpolation, there exist an axis which is reversed and an axis which is not reversed, at the same time. Therefore, the synchronousness of the actual movement of each axis is lost to cause an error to be produced in the motion locus. Resultantly, there causes such a problem that overcutting or undercutting may occur in a workpiece to cause the shape accuracy such as roundness, etc. of a product to be deteriorated.
As regards a problem of lost motions, such a method as to balance the lost motion quantity is employed at present, in which a correcting quantity is overlapped to the instruction values given to servo motors for two kinds of feedrates G0 and G1 when the feed orientation of the feed drive axis is reversed. General NC equipment is provided with such a correcting feature. Usually, this feature is called a backlash correcting feature. However, if the lost motion quantity is changed according to various running conditions since the correcting value is generally fixed in this correcting method, a sufficient correcting effect is not able to be obtained, or excessive correction is performed, whereby the machining accuracy may be further worsened.
Furthermore, an optimal backlash correcting method for various feedrates has been proposed (by, for example, Japanese Patent Publication No. Hel-7-71781). However, since the lost motion quantity is not determined by only the feedrate, a sufficient effect is not able to be expected with only this method.
It is therefore an object of the invention to provide a dynamic error correcting system which is able to interpolate synthetic dynamic errors of an NC machine tool and to maintain the machining accuracy by giving corrections to thermal displacements of the spindle head and the feed drive system, and the lost motions.